Reinforced concrete is a widely-accepted material of construction. Within the past 30 years, severe deterioration of many reinforced concrete structures has been observed with increasing frequency throughout the United States. Over an approximately 5-year period, the Federal Highway Administration adjusted its estimate of repair costs due to concrete deterioration in transportation systems alone from $50 billion to $700 billion ("The Status of the Nation's Highways and Bridges: Conditions and Performance and Highway Bridge Replacement and Rehabilitation Program 1989," Report of the Secretary of Transportation to the United States Congress, U.S. Government Printing Office, Washington, June 1989). This problem is widespread, affecting not only transportation infrastructure components, but also "snowbelt" structures, as well as coastal buildings and related structures. The safety and longevity of such reinforced concrete structures are prime concerns, and methods of protection are therefore important.
The basic problem associated with the deterioration of conventional reinforced concrete due to corrosion of embedded reinforcement is generally not that the reinforcing material itself is reduced in mechanical strength, but rather that the concrete cracks. Until now, it has been assumed that the products of corrosion exert stresses within the concrete which cannot be supported by the limited plastic deformation of the concrete, and therefore cracking of the concrete occurs. This was based on the erroneous assumption that the corrosion products from the steel occupy a relatively high specific volume within the concrete matrix and that this causes tensile stresses which lead to concrete cracking and spalling.
Cracking of concrete can lead to problems regarding structural soundness (on, for example, pilings), to discomfort (for example, chuckholes in bridges), or to cosmetic problems (as in the case of facades on buildings). Since concrete that has reached this state of deterioration is frequently extremely difficult to rehabilitate, significant effort has been expended to develop techniques capable of detecting the corrosion at an earlier stage.
Concrete contains pores which are interconnected throughout it, and this extensive network leads to permeability of the concrete to both liquids and gases. This is of critical importance in the corrosion process, because both the initiators (generally, chloride ion) and supporters (for example, oxygen) of corrosion of the reinforcing steel must diffuse through the overlying concrete to the steel.
A major influence that the composition of concrete exerts on the environment of any reinforcing steel which is placed within it is a relatively high pH. The pH appears to be governed by the free sodium, potassium, and calcium hydroxides within the concrete, which gives a pH somewhat above 12. There have been suggestions that the ultimate agent governing pH is, in fact, alkali content of the cement, because the pH of a saturated calcium hydroxide solution (pH 12.6) is lower than that observed from concrete pore water which has been "squeezed out" of hardened cement, mortar, and concrete.
It is believed in the art that various combinations of, and interactions between, geothite (.alpha.-FeOOH), magnetite (Fe.sub.3 O.sub.4), lepidocrocite (.gamma.-(8FeOOH,FeOCl)), and hydroxides and chloride containing hydroxides (2Fe(OH.sub.2 .multidot.FeOHCl.multidot.Fe(OH).sub.2 Cl) comprise the species at or near the embedded metal surface, with the relative amount of each varying with Cl.sup.-- /OH.sup.-- and temperature. Upon exposure to air and complete oxidation, iron reacts to Fe(OH).sub.3 or Fe.sub.2 O.sub.3. Experiments in which Ca(OH).sub.2 solutions were titrated with FeCl.sub.2, and in which pH versus time was maintained for similar solutions of differing Cl.sup.-- /OH.sup.-- ratios, have reproduced the actual pH transition that occurs in concrete or mortar (Grimes, W. D., W. H. Hartt, D. H. Turner [1979] Corrosion 35:309; Raharinaivo, A., J.-M.R. Genin [1986]Materiales de Construccion, Vol. 36, Oct., Nov., Dec., p. 5; Clear, K. C., Y. P. Virmani [1983] "Solving Rebar Corrosion Problems in Concrete Research Update: Methods and Materials," Paper No. 4, Proceedings of Seminar on Solving Rebar Corrosion Problems in Concrete, Natl. Assn. Cor. Engrs.).
Grimes et al. (Grimes et al [1979] supra) and Slater (Slater, J. E. [1983] Corrosion of Metals in Association with Concrete, STP 818, Am. Soc. for Test. and Matls., Phila., p. 5) have rationalized the tendency for solid corrosion products to crack concrete to the protectiveness of oxides formed during high temperature treatment, as quantified historically by the Piling-Bedworth ratio (Piling, N., R. Bedworth [1923] J. Inst. of Metals 29:534). On this basis, the specific volume of the oxide is compared to that of the metal from which it is formed, with oxide spalling tendency (reduced protectiveness) increasing in proportion to the ratio of the volumes (oxide-to-metal).
Certain additives have been used in attempts to improve the performance of reinforced concrete. Latex-modified concrete essentially uses a polymer emulsion in the mix-water which apparently impedes the penetration of surface chlorides into the concrete (and possibly oxygen diffusion through the concrete as well). Incorporation of wax compounds has also been used to create "internally sealed" concrete. In this approach, a heating cycle is employed following curing of the cement to came a hydrophobic layer of wax to form on the pore walls, which prevents ingress of surface chemicals.
The foregoing attempts, however, are directed to preventing the corrosion of embedded steel. These are in response to the view that accumulation of corrosion product in the concrete pore space produces tensile hoop stresses and, ultimately, cracking and spalling. There have been virtually no experimental results contradicting this proposed mechanism, the corrosion product/mechanical pressure (CPMP) cracking mechanism, in the past 80 years; and very little data has been developed to provide confirmation of the CPMP cracking mechanism. In contrast, the subject invention focuses on a different mechanism of concrete cracking and spalling from that which has been accepted for most of this century.